Zoom lenses, such as those used single-lens reflex cameras, employ mechanical cam systems to position variable-magnification lens groups, i.e., lens groups that move along the optical axis during changes in magnification ("zooming").
A prior art mechanical cam zoom mechanism ("cam") 5 for positioning lens groups in a zoom lens is shown in FIG. 1. Cam 5 includes a pin p1, which protrudes from a first lens group retaining member (not shown) and passes through a zoom cam track 8 and a linear cam track 12. The latter is formed so as to extend in a direction parallel to the optical axis A of the zoom lens. A pin p2 protrudes from a second lens group retaining member (not shown) and passes through a zoom cam track 14 and linear cam track 12. Zoom cam tracks 8 and 14 are formed to correspond to the zoom trajectories of first and second lens groups G1 and G2, respectively. Cam 5 is designed to allow linear cam track 12 to move parallel to itself, as indicated by double-arrow 18, between positions 22 and 24 when the zoom lens is in the extreme wide-angle state and the extreme telephoto state, respectively.
In the intermediate focal length state (i.e., when cam track 12 is between positions 22 and 24, as is shown in FIG. 1), the positions of pin p1 and p2 correspond to the position of the first lens group G1 and second lens group G2, respectively.
With reference now to FIG. 2, zoom lens 50 shows zoom trajectories 52 and 54 associated with first and second lens groups G1 and G2, respectively, for cam 5 of FIG. 1. The case shown in FIG. 2 is when there is no positional misalignment between the actual image plane IPW.sub.1 in the extreme wide-angle state (W) and the actual image plane IPT.sub.1 in extreme telephoto state (T) relative to reference image plane IPR. Reference image plane IPR is the image plane contemplated during design, and is where photosensitive film, a CDD array, or other image sensor resides. Image planes IPW.sub.1, IPR and IPT.sub.1 coincide when the focal lengths of lens groups G1 and G2, the distances between respective lens groups, and the profiles of the respective cam tracks all possess their design values.
In a zoom lens as actually manufactured, however, errors arise. For example, the focal lengths of the lens groups and the distances between respective lens groups differ from their design values due to errors in radii of curvature of lens surfaces, distances between lens surfaces, lens thicknesses, refractive indices of lens materials, and so forth. In addition, there are also machining errors in the profiles of the respective cam tracks. With reference now to FIG. 3, zoom lens 60 of FIG. 3 is the essentially the same as zoom lens 10 of FIG. 2, except that zoom lens 60 has the above-described manufacturing errors, and also suffers from being assembled without any adjustment. In this case, zoom trajectories 52' and 54' for zoom lens 60 will differ from their ideal trajectories 52 and 54 of zoom lens 50. Thus, for zoom lens 60, the actual image plane IPW.sub.2 in the extreme wide-angle state (W), the actual image plane IPT.sub.2 in the extreme telephoto state (T) and the reference image plane IR do not coincide.
In a conventional zoom lens, such as zoom lens 50 of FIG. 2, focusing is carried out by causing the first lens group G1 (i.e., the most objectwise lens group) to move along axis A (i.e., move axially). When employing this focusing method, a helicoid mechanism (not shown) is provided for the first lens group G1, which accommodates the respective states for filming objects at different distances i.e., from an infinite-distance focus state to a short-distance focus state. Such focusing is achieved by appropriately changing the angular displacement of the helicoid mechanism and controlling the amount of focusing movement of first lens group G1. The location of reference image plane IPR is defined based on the location of a mounting reference plane (not shown) for mounting film, CCDs, etc., provided on the lens barrel (not shown). The mechanism is designed such that the position of the mounting reference plane with respect to the principal structures (cams, etc.) of the zoom lens can be adjusted by means of washers (i.e., shims).
With reference now to FIG. 4, measuring apparatus 70 measures the position of image plane IP of a "target" zoom lens 74 to which adjustment is to be carried out using conventional zoom lens adjustment methods. Measuring apparatus 70 comprises, in order along an optical axis A1, a light source 80 for generating a diverging light beam 82, a slit 84, a half-mirror (i.e., beam splitter) 86, a collimating lens 88, and a mirror 90 with a reflective surface 92. Also included in apparatus 70 along an axis A2 which intersects axis A1 at half-mirror 86, is an ocular 94 and a reticle 96. The above-mentioned elements, except for mirror 90, constitute a collimator 100.
In measuring apparatus 70, light beam 82 from light source 80 passes through slit 84, is incident half mirror 86 and passes therethrough to collimating lens 88. The latter converts diverging light beam 82 into a collimated beam 104, which is equivalent to light from an object at an infinite distance. Collimated light beam 104 is then incident target lens 74, which transforms the collimated light beam into a converging light beam 106. The latter is incident reflective surface 92 of mirror 90 arranged in the vicinity of image plane IP of target lens 74.
Mirror 90 is made to move back and forth along optical axis A1 to measure the position of image plane IP. When reflecting surface 92 of mirror 90 coincides with the position of image plane IP, then converging light beam 106 is reflected from surface 92, thereby forming a diverging light beam 106' which travels back along the path of converging light beam 106. Diverging light beam 106' passes back through target lens 74, which forms a collimated light beam 104' which travels back along the path of collimated light beam 104. Collimated light beam 104' is then incident collimator lens 88, which forms a converging light beam 82', which travels back along the path of diverging light beam 82. Converging light beam 82' proceeds to half mirror 86, which reflects converging light beam 82' to form an image on reticle 96.
When the position of reflecting surface 92 of mirror 90 is shifted along optical axis A1 such that it no longer coincides with the position of image plane IP, then diverging light beam 106' is not reflected back precisely along the path of converging light beam 106. Consequently, light beam 104' traveling back toward light source 80 will not be collimated, i.e., will be a divergent light beam or a convergent light beam. Accordingly, the image formed on reticle 96 will be observed by an observer 110 to be out of focus.
With reference also to FIG. 3, using measuring apparatus 70 of FIG. 4 and the conventional image plane position measuring technique described above, the position of actual image plane IPW.sub.2 when in the extreme wide-angle state and the position of the actual image plane IPT.sub.2 when in the extreme telephoto state can be measured by the appropriate adjustment of the lens groups. As a result, it is possible to determine the amount of positional misalignment d between actual image planes IPW.sub.2 and IPT.sub.2.
With reference now to FIG. 5, a lens L lying along optical axis A includes an object point O, an image point I, a front focus F, a back focus F', a focal length f, and a transverse magnification .beta.. Also, x is the axial distance from front focus F to object point O, and x' is the distance from back focus F' to image point I. Imaging by lens L is governed by Formula (1) (Newton's equation): EQU xx'=-f.sup.2. (1)
Transverse magnification .beta. of lens L is defined by Formula (2): EQU .beta.=(x'+f)/(x-f). (2)
Using the relationships in Formulas (1) and (2), above, the transverse magnification .beta. can be expressed as Formula (3): EQU .beta.=f/x. (3)
If object point O in FIG. 5 moves axially by an amount .DELTA.x, then image point I will move axially by a corresponding amount .DELTA.x'. In this case, from Newton's equation, the relationship in Formula (4) obtains: EQU (x+.DELTA.x)(x'+.DELTA.x')=-f.sup.2. (4)
The amount of movement .DELTA.x' of image point I is given by Formula (5): EQU .DELTA.x'=(f.sup.2 .multidot..DELTA.x)/{x(x+.DELTA.x)}. (5)
Accordingly, the longitudinal magnification .alpha.=.DELTA.x'/.DELTA.x of the lens can be approximated, for small movements .DELTA.x of object point O, by Formula (6): EQU .alpha.=f.sup.2 /{x(x+.DELTA.x)}.apprxeq.(f/x).sup.2 =.beta..sup.2. (6)
With reference now to FIG. 6 and zoom lens 120, the front focus of first lens group G1 corresponds to an object point O2 of second lens group G2. If first lens group G1 moves along axis A by an amount .DELTA.x, then the object point O2 likewise moves by the same amount .DELTA.x. Accordingly, when first lens group G1 is made to move axially by an amount .DELTA.x, the amount of movement .DELTA.x' of image plane IP of the zoom lens is given by Formula (7): EQU .DELTA.x'=.DELTA.x.multidot..beta..sup.2 (7)
Furthermore, the focal length f of the entire zoom lens is given by Formula (8): EQU f=f1.multidot..beta.. (8)
Here, f1 is the focal length of first lens group G1, and .beta. is the combined transverse magnification produced by second lens group G2 and any lens groups downstream (i.e., imagewise) thereof. In a two-group zoom lens, such as zoom lens 120, transverse magnification .beta. is that of second lens group G2.
In a zoom lens such as zoom lens 120, the value of transverse magnification .beta. due to the second lens group G2 and any lens groups downstream thereof, will vary as a function of the focal length state (i.e., as a function of the "zoom position"). For this reason, a constant amount of movement .DELTA.x of first lens group G1 will not produce a constant amount of movement .DELTA.x' of image plane IP. Rather, the amount of movement .DELTA.x of image plane IP will vary as a function of the focal length state. It is thus useful to define the ratio K1W=.DELTA.x'/.DELTA.x, which is the ratio of the amount of axial movement .DELTA.x' of image plane IP with respect to the amount of movement .DELTA.x of first lens group G1 when in the extreme wide-angle state. The ratio K1W represent an "image plane movement index" for first lens group G1 when in the extreme wide-angle state.
Likewise, it is useful to define the ratio K1T=.DELTA.x'/.DELTA.x, which is the ratio of the amount of axial movement .DELTA.x' of image plane IP with respect to the amount of axial movement .DELTA.x of first lens group G1 when in the extreme telephoto state. The ratio K1T represents an image plane movement index for first lens group G1 when in the extreme telephoto state.
With reference now to FIG. 7, conventional adjustment method for zoom lens 120 is carried by "zooming adjustment," which is an asymmetrical adjustment. This method involves exploiting the difference between the image plane movement indices K1W and K1T and the elimination of the positional misalignment d between the actual image planes IPW.sub.2 and IPT.sub.2. In zooming adjustment, first lens group G1 is made to move by an amount .DELTA.x such that the relationship indicated in Formula (9) holds: EQU .DELTA.x(K1t-K1w)=d (9)
Zooming adjustment by moving first lens group G1 by an amount .DELTA.x makes it possible to cause the actual image planes IPW.sub.2 and IPT.sub.2 to coincide at image plane IP. However, there will still be a positional misalignment D between image plane IP and the reference image plane IPR following zooming adjustment. In the conventional adjustment method, the positional misalignment D between the image plane IP and reference image plane IPR is adjusted by what is called "back adjustment." This involves changing the thicknesses of washers (not shown) that define the location of the mounting reference plane (not shown). This back adjustment is equivalent to making the entire zoom lens 120 move forward or backward along optical axis A.
In video cameras and electronic still cameras employing solid-state image sensors such as CCDs, it is desirable to be able to reduce the size of the zoom lens to keep up with the increasing miniaturization of the image sensor. However, with conventional zoom lenses, the lens barrel needs to be large enough to allow for a mechanical cam mechanism, such as described above. Accordingly, it has been difficult to achieve a suitably small zoom lens with a mechanical cam suitable for a video camera, electronic still camera, or the like employing ever-smaller solid-state image sensors.
With solid-state image sensors such as CCDs, individual differences arise in the distance between the mounting reference plane of the solid-state image sensor package and the reference image plane of the solid-state image sensor itself (i.e., this distance will vary from sensor array to sensor array). This makes it necessary to perform zoom lens adjustment after the image sensor package has been securely arranged on the zoom lens. In this case, with the conventional zoom lens adjustment method using measuring apparatus 70 employing collimator 100 and mirror 90 (See FIG. 4), the image sensor is securely arranged at the location at which mirror 90 would need to be installed. Thus, to date, it has not been possible to determine the locations of the actual image planes when in the extreme wide-angle and extreme telephoto states, making conventional adjustment of the zoom lens impossible.